This application relates to an apparatus and method for providing snapshot action thermal infrared imaging within automated process control article inspection applications. More specifically, it pertains to the use of snapshot mode lead salt area-array imaging sensors as the imaging front-end in high-speed machine vision inspection systems. The use of lead salt-based image acquisition sensors in a snapshot or stop-action mode allows for a wide variety of automated process control or article inspection applications.
While the invention is particularly directed to the art of high-speed automated product inspection, and will be thus described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications. For example, the invention may be used in other applications where snapshot action is desired in a high-speed control process.
By way of background, the use of camera or video-based automated inspection techniques to aid in the quality or process control operations of manufacturers or producers is well known in the art. For example, U.S. Pat. No. 4,760,444 entitled “Machine Visual Inspection Device and Method” describes a device and method related to camera-based machine visual inspection of toothbrushes. This patent discusses the use of a fluorescent lamp to illuminate the target. It also references the camera measuring the reflectance data of the inspection field/toothbrush. As a further example of the state-of-the-art, U.S. Pat. No. 4,882,498 discusses the use of pulsed LED illumination to inspect specimens.
Typical in many of the prior art implementations of automated inspection, charge-coupled-device (CCD) based cameras are used. For example, U.S. Pat. No. 4,875,100 entitled “Electronic Shutter for a CCD Image Sensor” describes an innovation to the prior art architecture of CCD devices which extends its functionality to include an electronic shutter capability. This additional function allows CCD-based video imagers to provide “stop action” functionality where the action is occurring in rapidly changing image scenes without the need for synchronized, pulsed illumination. Many subsequent implementations of automated machine vision systems are known which are CCD-based.
The need to limit and synchronize in time the acquisition of imaged signal photons is a fundamental requirement of high-speed automated inspection. This statement is true independent of the wavelength of operation of the chosen image sensor used in a particular machine vision application. Cameras that are operable in the thermal IR region of the electromagnetic spectrum (between 2 and 12 um) and possess the ability to stop the action of rapidly evolving spatial scenes are currently known to exist. For example, the Radiance HSX High Performance Imaging Camera manufactured by Raytheon Corporation is purported to support snapshot mode acquisition. There are, however, several drawbacks to using this type of camera in machine vision applications and thus commercial deployment of thermal IR cameras in machine vision applications has been difficult. These drawbacks include the high-price and lack of ruggedness associated with a Stirling-cycle cooled, Indium Antimonide-based camera of this type.
A relatively new class of thermal infrared cameras/sensors has recently become available. This class of infrared imagers can generically be described as microbolometer-based devices. Key attributes of microbolometer-based thermal IR imagers include the fact that they do not require cryogenic cooling in order to operate as well as the fact that they can be fabricated using standard silicon CMOS IC fabricating equipment and processes. These attributes help to eliminate the high cost and lack of ruggedness associated with historical infrared imaging techniques. U.S. Pat. No. 5,021,663 entitled “Infrared Detector” and U.S. Pat. No. 5,286,976 entitled “Microstructure Design for High IR Sensitivity” describe the construction of suitable bolometer pixel-sites and pixel array deployments suitable for the detection of spatially varying thermal infrared signals. U.S. Pat. No. 5,489,776 entitled “Microbolometer Unit Cell Signal Processing Circuit” shows how other signal processing functions (in addition to the signal detection function) can be incorporated on a per-pixel basis using standard and well known CMOS fabrication processes. This patent describes the use of capacitor and transistor components as well as electrical interconnects all fabricated within the silicon-based microbolometer cell. These pixel site structures are used to convert the received thermal energy into ordered electrical signals representative of the scene that has been imaged onto the surface of the microbolometer array. U.S. Pat. No. 5,420,419 entitled “Camera for Producing Video Output Signal, Infrared Focal Plane Array Package for Such Camera, and Method and Apparatus for Generating Video Signals from Passive Focal Plane Array of Elements on a Semiconductor Substrate” describes one manner in which existing microbolometer imaging arrays can be deployed within an operating infrared camera.
Despite the body of knowledge that is available, none of the state-of-the-art microbolometer devices have been implemented in a manner that addresses the specific pixel-level functionality required for snapshot mode image acquisition. In addition, the inherent pixel-site response time and measurement sensitivity associated with microbolometer technology does not currently meet the requirements associated with very short signal integration, snapshot mode acquisition.
Single element lead salt detectors, in particular lead sulfide (PbS) and lead selenide (PbSe), have been used for decades in various applications involving the detection of infrared energy. Useful detection sensitivities are possible when one operates lead salt detectors at room temperature. Improved sensitivity is achieved as the device operational temperature is lowered and, in many applications involving lead salt detectors, 1 or 2 stage thermo-electric coolers are used.
However, the use of costly and undesired cryogenic cooling is not required when using this class of thermal IR detector. Moreover, in recent years, the ability to closely merge or integrate active two-dimensional arrays of lead salt photosites with silicon-based control electronics has made it possible to begin fabricating large area two-dimensional IR imaging arrays out of PbS and PbSe.
The lack of a cost-effective snapshot mode thermal IR camera possessing adequate temperature resolution performance without requiring cryogenic cooling has limited the application of thermal IR imaging front-ends within state-of-the-art machine vision systems. The disclosed invention contemplates a system and method for overcoming the known limitations.